Electric cable with air space insulation



' Oct. 4, 1938. E. swm ET AL 2,131,987

ELECTRIC CABLE WITH AIR SPACE INSULATION Filed Oct. 11. 1953 Pb; YsfykoLI z 3 s 7 l/mPaev/zc' Patented Oct. 4, 1938 CABLE WITH Am SPACEINSULATION I Ernst Studt and Carl Traugott, Nordenham, Germany,assilnors to Norddeutsche Seelrabelwerke 'Aktiengesellsch'aft,Nordenham, Germany Application October 11, 1933, Serial No. 693,212

In Germany 4 Claims.

It is known toreduce the capacity between the conductors of a telephonecable by surrounding the copper conductors with a paper and air spaceinsulation, by first placing paper cord round the copper conductors andthen winding thereon a covering of paper bands. By means of suchinsulated conductors it is possible to produce multiccnductor cables,between the conductors of which the dielectric constant of thedielectric is 1.6.

According to the inve tion the air space insulation of the conductors ofthe signalling cables is constructed of polystyrol, more particularly bymeans of threads, cords or bands of polystyrol, which are provided onthe conductors, in the same way as paper threads, cords and bands. Suchinsulated conductors used either alone or stranded with other conductorsinto pairs, quads ,or groups of conductors of higher order, form thecable core, over which there is pressed an enclosing water-tightsheathing. The air space insulation may be constructed, moreparticularly in the case of land cables, by the simultaneous use ofcords and bands of paper and polystyrol, a cord of polystyrol beingplaced round the copper conductor and a paper band-winding round thesaid cord. The threads, cords and bands of polystyrol may be suitablyused in a profiled form.

The properties which render pure polystyrol especially suitable for theformation of air space insulated conductors, as compared with otherinsulating substances, are its small dielectric constant (e=2.5), itsvery small angle of loss (tan. 6=0.2 10 its inappreciably small waterabsorptionand its good mechanical resistance.

An air space insulation constructed according to this invention has adielectric constant which is at least equal to that of paper and airspace insulation, Further, the small angle of loss of the polystyrolproduces only small dielectric-losses which, more particularly asregards high frequencies, are considerably less than the dielectric lossof paper and air space insulation. This is of special importance in thecase of long land and submarine cables, since, as is well known, thereis a gradual tendency to utilize the lines in a multiple way, byvemploying carrier frequencies, and since, in the case of small specific.

damping, the influence of the leakage is very noticeable. This is alsoof great importance in the case of cables for broadcasting andtelevision, wherein use is made of comparatively high frequencles.

Further advantages of the air space insulation October 24, 1932 (Cl. li-265) constructed by means of polystyrol in contradistinction to paper,result from the propertyof the polystyrol of not being hygroscopic. Twoimprovements in the manufacture and in the mechanical construction ofthe signalling cable 5 resulting from this property are to be especiallyreferred to. As is well known, it is necessary to dry a paper and airspace insulated cable before it is finally completed, in order to removethe moisture from the paper. This drying re- 10 quires, more especiallyin the case of long cables,

very expensive methods and apparatus. If according to this invention useis made of polystyrol for the construction of the air space insulation,it' is possible to dispense with these drying processes, since thenon-hygroscopic polystyrol does not contain or adsorb any moisture.

A further advantage which results from this property of polystyrolconsists in that it becomes possible, in the case of air space insulatedsubmarine cables, to replace the lead sheathing by a covering ofgutta-percha or mixtures of polystyrol, gutta-percha, balata, pure or inadmixture with caoutchouc, wax or like substances; The endeavours to usea covering of gutta-percha with air space insulated submarine cableshave hitherto failed because the hygroscopic paper gradually drawsmoisture from the gutta-percha covering, whereby its electric propertiesare impaired. For this reason submarine cables, which are paper and airspace insulated, have always been provided with an enclosing leadsheathing. By using polystyrol according to the invention this is nolonger necessary. It is on the contrary possible to use especially lightcoverings of guttapercha and similar substances which caneasily beapplied as an outer enclosing covering of the cable. The reduction inweight thereby secured is of great advantage in the manufacture andlaying of submarine cables.

Owing to the great resistance of the polystyrol the air space insulationconstructed according to this invention is considerably more resistantto pressures on all sides than paper insulation manufactured in the sameway. Whilst cables hav- 4 ing conductors which are paper and airinsulatedrequire a pressure protecting covering even at water depths ofabout 70-meters, a cable with conductors according to this invention canbe used, as has been found by experiments, without 50 any pressureprotecting covering at water depths of aboutlOOO meters.

Hitherto, spacers provided on the conductors have generally consisted ofcords and threads. According to one mode of carrying the invention 66into effect use is made of tubular threads of polystyrol for theproduction of an air space insulation, instead of solid threads orcords.

These tubular threads are wound round the cop per conductors. Over thiswinding, there is then provided, for the completion of the insulation, aband winding or a hose-like covering, for instance of polystyrol.

Tubular threads of polystyrol can be manufactured with a very small wallthickness and yet possess throughout sufficient resistance to pressureso that an air space insulation constructed by means of tubular threadshas a very small dielectric constant and the angle of loss is far belowthe value hitherto reached.

In order to render the insulation resistant also with respect to highouter pressures, with which the air space insulation constructed ofthreads, cords or bands as above referred to no longer can cope, withoutat the same time substantially impairing the dielectric properties,according to this invention, the air spaces between the individual turnsof the winding are filled up by powdered or granular insulatingsubstances of high dielectric values. Use may be made for this purposeof insulating ceramic materials, of powdered polystyrol, or of ambercomminuted to powder. The powdered insulating material may be used indifferent degrees of fineness, according to the filling factor that isconsidered suitable in each particular case, for mechanical ordielectric purposes.

A further development of the invention relates to the construction ofthe air space insulation in cables with concentric arrangement of theconductors as used in high frequency transmissions, for instance incarrier frequency telephony or picture transmission. In the building upof the air space insulation in such concentric arrangements ofconductors use is made of spacers between the inner and the outerconductor, which spacers consist of a material having high insulatingproperties and a dielectric constant, as small as possible.

According to the invention, the spacers are made of polystyrol. Ascompared with the most usual insulating substances used in theelectrical industry, polystyrol has the special advantage that in thepresence of moisture, its surface is not coated by a continuous layerthereof. The surface resistance of polystyrol, therefore, is notsubstantially reduced even when condensation water from the surroundingmoist air is deposited on its surface with'a change in temperature,since the moisture resolves itself into a number of individual smalldrops which are separated from one another by zones of high surfaceresistance. This is of special importance in the case of concentricarrangements of conductors, since the spacers of a line which hasalready been laid, as a rule is no longer directly accessible so that itis very desirable, when the line is being constructed, to use spacers ofhigh quality, the insulating capacity of which is to a. large extentindependent of weather conditions and of other phenomena which wouldinfluence the moisture between the conductors.

The accompanying drawings illustrate, by way of example, severalconstructions with air space insulation constructed by means ofpolystyrol.

In the drawings:

Fig. 1 is a perspective view of one form of the cable constructed inaccordance with the invention.

Fig. 2 is a perspective view illustrating a modifled spacer element.

gether. The copper conductor I is surrounded by the thread 2 woundthereon in an open helix, a covering band winding 3 being placedthereon. Both, thread and band, are made of polystyrol. Four conductors,each constructed as just referred to, are stranded to a quad 4 and heldtogether by a band of polystyrol 5. Four such quads stranded togetherform the cable core, which is surrounded by the water-tight sheathing 6and the armouring '1.

Alternatively, the individual air space insulated conductors of thecable. may be insulated in the manner shown in Figure 2. The copperconductor I is surrounded by a tubular thread 8, acting as a spacer,over which the covering 9 is placed, which consists of a tube ofpolystyrol.

Figure 3 illustrates a further modification of the air space insulationwhich is especially pressure resistant. As in the case of Figure l thecopper conductor is surrounded by a thread and a band winding, 2 and 3respectively. A granular mass it], e. g. granular polystyrol, is placedin the gaps between the turns of the thread 2, the said mass preventingthe insulation from being compressed.

Fig. 4 illustrates a modification of the invention as applied toconcentric constructions of conductors.

In this figure, the outer tubular conductor l l is supported by aprofile band i5 of polystyrol serving as spacer and laid with a longpitch around the inner conductor I3.

It is possible to make bands and threads of polystyrol in such a mannerthat they are flexible.

It has, however, been found that the polystyrol bands and threads whichare otherwise crystal clear when they are wound on small diameters areeasily clouded and have a milky appearance, from which it is concludedthat the structure is too strongly stressed. Moreover, the resilientforce inherent in the polystyrol bands and threads prevents the threadwinding from lying firmly on the copper conductor and causes it to havea tendency to unwind and spring apart.

In order to avoid this, according to the invention, the bands and cordsof polystyrol or like artificial substance are heated to about 70-80centigrade, before they are wound on. This may be eifected either bypassing the threads or bands, between the storage reel and the windingon device, through a heated tube or, for instance in the manufacture ofcovered copper conductors, by arranging the Whole of the winding device,including the storage reel, in a heated container.

When the bands and threads of polystyrol attain a. temperature of 70-80centigrade, they can be placed around the conductor without anyresistance and they remain clear as crystal.

A drawback of the method described consists in that the tensionalresistance of the threads decreases with increasing heating, so that thethreads can be wound on only at a. low speed it the danger is to beavoided that the threads will break or that their thickness will changeduring the winding on. In order to overcome this drawback, according toanother feature of the invention, as shown in Fig. 5 the threads 2i areprovided with an insertion of tension resisting material, for instancehemp or wire 22. Such threads can be wound on at great speed also wheniil in a heated condition, without breaking or without changing theirthickness.

If small dielectric losses of the conductors have first to be taken intoconsideration in the manuiacture of air space insulated conductors, itis an advantage to use metallic insertions in the threads of artificialsubstances. It is true that in this way the capacity of the conductorsduring working is slightly increased. The position is just 'the oppositeit non-conducting insertions, such as cotton yarns or hemp, are used.These do not cause an increase in the capacity during working but asmall increase in dielectric losses.

In any case, however, the deterioration of the electrical values remainswithin very small limits.

Having now described our invention, what we claim as new and desire tosecure by Letters Patent is:

1. In an electrical conductor with air space insulation, 8. centralmetallic wire, an open helix of polystyrol wound around the metal wire,and a covering surrounding said wire and helix, the helix thus forming aspacer between said covering'and the metallic wire and providing the airspacing, and the helix of polystyrol' being in the form of a tube.

2. In an electrical conductor with air space insulation, a centralmetallic wire, an open helix of poly'styrol wound around the metal wire,and a covering surrounding said wire and helix, the helix thus forming aspacer between said covering and the metallic wire and providing the airspacing, and the helix oi polystyrol being in the form of a tubularthread with a tension-resistance insertion provided in the interior ofsaid tubular thread.

3. In an electrical conductor with air space insulation, a centralmetallic wire, an open helix of polystyrol wound around the metal wire,and a" covering surrounding said wire and helix, the helix thus forminga spacer between said covering and the metallic wire and providing theair spacing, and a granular insulating substance in the free spacesbetween the turns of the spacer.

4. In an electrical conductor with air space insulation, a centralmetallic wire, an open helix of polystyrol wound, around the metal wire,and a covering surrounding said wire and helix, the helix thus forming aspacer between said covering and the metallic wire and providing the airspacing, and a granular insulating substance in the free spaces betweenthe turns of the spacer consisting of powdered polystyrol.

ERNST STUDI. cam. raaoeorr.

